docs: def output_story_prefixes(self):
          """ Writes the set of prefixes to a file this is useful for pretty
          printing in results.latex_output. """
  
          if not self.test_story:
              raise NotImplementedError(
                  "I want to write the prefixes to a file"
                  "called <test_story>_prefixes.txt, but there's no test_story.")
  
          fn = os.path.join(TGT_DIR, "%s_prefixes.txt" % self.test_story)
          with open(fn, "w") as f:
              for utter_id in self.test_prefixes:
                  print(utter_id.split("/")[1], file=f)